The present invention relates to a device for measuring axle geometry at the wheel axles of motor vehicles with the wheels turning, the device having at least two pairs of driven rotating cylinders that are positioned essentially away from the vehicle track and that the front or rear wheels of the vehicle can rest on, each pair positioned in a frame that can move in a horizontal plane in relation to a base plate, with a sensing component that can be positioned against the side of a wheel resting on a pair of the rotating cylinders and that is connected to a goniometer that determines the position of the sensing component in relation to a reference system that is fixed with respect to the device.
A device of this type is described, for example in German OS No. 2 204 918.
The frame in the known device is positioned in such a way that it can rotate in the horizontal plane and if necessary be shifted transversely in relation to the motor vehicle. The sensing component is mounted on the frame and, when it is applied to the side of the vehicle wheel, the force of reaction that acts on the sensing component adjusts the pair of rotating cylinders perpendicular to the frame. The torsion of the frame is then measured to determine the amount of toeing in. King-pin angle, castor length, and steering-knuckle pivot inclination, are determined by goniometers mounted on the sensing component.
The drawback of the known device is that it can only supply fairly precise results when the tire cap has an ideal shape. When the cap is slightly conical for instance, zero-force measurement becomes impossible because the conicity will generate transverse forces that also act on the cylinder pair and contaminate the results. Furthermore, another sensing component has to be applied to the inner side of the wheel to prevent it from getting displaced to the side. Applying a second sensing component of this type against the inner side of a wheel that is mounted on a vehicle, however, entails considerable problems in design because there is usually not enough space available.
Other devices that measure the axle geometry of a motor vehicle by sensing the orientation of the side of a stationary wheel are known. The vehicle rests on floating pairs of rotating cylinders that both enable unstressed measurement and allow the wheels to be turned 180.degree. to eliminate rim runout.
These devices, however, cannot be employed to measure motor-vehicle wheels that have significant irregularities, raised lettering for example, on the sidewalls of the tires.
Still other devices are known that obtain measurements by sensing the sidewalls of the tires with the wheels turning on cylinder pairs that are fixed in position. Wheel irregularities like the aforesaid raised lettering are eliminated by the inertia of the measuring system.
The vehicle, however, must be secured to prevent lateral migration, and the finite vehicle track necessarily leads to deformation of the wheel suspension and hence defective results.
Finally, devices are known that exploit the forces of reaction (the transverse forces of the turning wheel for instance) on the cylinder pair to orient the pair in relation to the wheel either directly or through servo systems in such a way that the reaction forces tend to zero.
The cylinder-pair angular positions in relation to a reference system that is fixed with respect to the device that result from the readjustment are directly exploited as a measure of such axle-geometry parameters as track and king-pin angle.
This method also has a drawback in that the results can be impermissibly contaminated by non-ideal (e.g. conical) tires and the resulting forces.